The present invention relates to an automatic transmission in a hybrid electric vehicle and more particularly to an automatic transmission in a hybrid electric vehicle where the engine shuts down when the vehicle stops.
Hybrid electric vehicles are vehicles that have a hybrid powertrain, typically including an electric drive system that is packaged in parallel with a conventional internal combustion engine. For hybrid electric vehicles with an internal combustion engine and an automatic transmission, it is preferable if the engine shuts down while the vehicle is stopped. By stopping the engine, the fuel economy of the vehicle is improved. However, the main transmission oil pump, which is typically driven by the rotation of the transmission input shaft, will stop pumping oil when the engine stops. But there is still a need for enough oil pressure to keep the clutches that are active in first gear (forward and low-reverse) stroked (i.e. engaged), so that the vehicle can immediately provide torque to the drive wheels when the vehicle operator demands a vehicle launch. If the oil pressure is not present, then the vehicle will not begin moving until the oil pressure can again build up to stroke the needed clutches. This would create an unacceptable delay for the vehicle operator.
Thus, it is desirable to have a hybrid electric vehicle with an automatic transmission, where the engine can shut down during a vehicle stop, but that also allows for immediate torque output when the vehicle operator demands a vehicle launch.
In its embodiments, the present invention contemplates a transmission for a hybrid electric vehicle. The transmission includes an input shaft with a main transmission fluid pump drivingly connected thereto, an electronically controlled hydraulic shift system, a low-reverse clutch and a forward clutch connected to the hydraulic shift system, and an electronically driven fluid pump. The transmission also includes a shut down shuttle valve operatively engaging the electrically driven fluid pump, the low/reverse clutch and the forward clutch and switchable to selectively engage and disengage the electronically driven pump from the hydraulic shift system.
The present invention further contemplates a method of operating a hydraulic system in an automatic transmission of a hybrid electric vehicle, with the transmission including an input shaft and a main pump for generating pressure in a hydraulic fluid when the input shaft is rotating, the method comprising the steps of: detecting that a rotational velocity of the transmission input shaft is below a predetermined threshold, and that a gear shift is in one of a drive and a low position; actuating an auxiliary pump to boost a pressure of the hydraulic fluid; operating the auxiliary pump to increase the hydraulic fluid pressure to a desired line pressure; hydraulically connecting the auxiliary pump to a low-reverse clutch and a forward clutch; and hydraulically disconnecting the main pump from the low-reverse clutch and the forward clutch.
An embodiment of the present invention allows for the addition of an electronically controllable transmission oil auxiliary pump, and a shut-down shuttle valve, so that there is enough oil pressure at the forward and the low-reverse clutches to allow for a quick vehicle launch, even when the vehicle engine is shut down while the vehicle is stopped.
An advantage of the present invention is that the internal combustion engine can shut down during a vehicle stop, thus improving the fuel economy of the hybrid electric vehicle.
Another advantage of the present invention is that the hybrid electric vehicle can begin transmitting torque to the drive wheels immediately upon driver demand for the torque, even when the engine is shut down during a vehicle stop.
A further advantage of the present invention is that the hydraulic pressure needed to obtain an immediate vehicle launch after the vehicle is stopped is produced with minimal additional hardware versus a conventional automatic transmission, and with minimal parasitic losses from generating the needed hydraulic pressure.